History has shown that La Niña years often bring twice as many floods to Vietnam as El Niño, endangering both agriculture and urban areas. Luu Le Huong of the Institute of Strategy and Policy on Agriculture and Environment examines the risks and the lessons for disaster preparedness.
Forecasts for the 2025-2026 La Niña show a high probability of occurrence between October and December at 70-71 per cent, easing to 54-70 per cent through the winter and dropping below 55 per cent in early 2026. The chance of returning to neutral conditions rises to 55-74 per cent, suggesting a short La Niña cycle that could still have significant impacts.
This coincides with the peak flood season in the Central region of Vietnam, creating a risk of an in increase in rainfall of 9-19 per cent compared with the multi-year average. Data show that La Niña years typically bring months of heavy rainfall 4.6 times more frequently than during El Niño phases.
The Central region, particularly provinces from Thanh Hoa to Khanh Hoa, with their short and steep rivers, is prone to flash floods and landslides. The Red River Delta may face urban inundation if heavy rains coincide with late-season typhoons.
In the Mekong Delta, compound flooding could occur as rising Mekong River levels overlap with high tides and monsoon-driven sea level rise, posing challenges for disaster management. This demands enhanced hydrometeorological forecasting, coordinated reservoir operations, and prepositioning of emergency resources to reduce losses.
The El Niño-Southern Oscillation cycle stands as the dominant factor shaping interannual variability in global climate systems, emerging from irregular interactions between atmospheric circulation and oceanic heat distribution across the tropical Pacific.
At its core, the cycle oscillates between El Niño, a warming phase that weakens trade winds and raises sea surface temperatures (SST) in the eastern Pacific, and La Niña, a cooling phase marked by intensified easterly winds and strong upwelling of cold water along the equator and the American west coast. Each phase generally persists for several months and repeats on a timescale of two to seven years, exerting profound influence on precipitation patterns, storm tracks, and hydrological extremes around the globe.
While El Niño tends to displace convection eastward and redistribute rainfall towards the central and eastern Pacific, La Niña strengthens the westward push of warm surface waters, invigorating rainfall across the western Pacific basin and amplifying flood risk in Southeast Asia.
By September 2025, multiple meteorological agencies, including the Philippines' DOST-PAGASA and the World Meteorological Organisation, converged in declaring heightened alert status for La Niña. The scientific consensus points towards a rapid onset in the October-December period, with probabilities surpassing 70 per cent.
This window of certainty highlights the urgent need for preparedness, as the climatic configuration strongly favours above-normal rainfall across vulnerable nations in the western Pacific. Forecasts further suggest persistence through the Northern Hemisphere winter, with probabilities moderating to the mid-fifties by early 2026 before a return towards neutral conditions.
While the event is projected to be relatively weak and short-lived compared to multi-year anomalies, the compressed timeline poses its own challenges, concentrating severe hydrometeorological impacts into a narrow seasonal window.
The projected intensity and duration have been characterized as weak and short compared to past events, such as the prolonged 2020-2023 La Niña.
However, scientific assessments stress that weak intensity at the global scale does not necessarily imply diminished local hazards.
Rising baseline global temperatures, accelerated by anthropogenic climate change, increase the energy available to atmospheric systems. This amplifies the likelihood of localised extremes during La Niña events, regardless of nominal classification.
Regions already experiencing marine heatwaves or anomalous thermal gradients may encounter non-linear amplification of convection, raising flood and storm risks far beyond what statistical analogues alone would suggest. The convergence of global warming with cyclical variability underscores the complexity of projecting impacts, demanding that governments avoid complacency based on expectations of weakness.
Mechanistically, La Niña reorganizes atmospheric circulation by strengthening the Walker Cell. The intensified east-west SST gradient forces convection over the western Pacific Warm Pool, producing enhanced rising motion, deep cloud development, and widespread rainfall over maritime Southeast Asia.
Reinforced easterly trade winds simultaneously drive warm water towards the west, elevating sea level by as much as half a meter in areas around Indonesia and the Philippines. This anomaly increases baseline coastal vulnerability and slows riverine drainage into the sea, prolonging inland inundation after heavy rain.
Compound hazards emerge as fluvial flooding coincides with storm surge and elevated sea levels, expanding both the depth and duration of inundation in densely populated deltaic regions such as the Mekong and Red River systems.
Regional monsoon dynamics further intensify the hazard profile. During transition seasons, La Niña enhances both southwest and northeast monsoon systems.
In late 2025, enhanced Habagat circulation continues to deliver moisture to western Philippines and Indochina, while the strengthening Amihan simultaneously drives rain towards eastern seaboards.
Vietnam's central provinces often experience peak rainfall during this period, with La Niña adding between 9 and 19 per cent to seasonal totals relative to climatology. The overlapping influence of cyclonic anomalies, moisture-laden winds, and reinforced monsoonal flows sets the stage for severe flooding, landslides, and agricultural disruption.
For archipelagic and coastal nations, the dual exposure of western and eastern flanks during the same quarter presents a uniquely difficult crisis management scenario, requiring national-level coordination to manage simultaneous emergencies across disparate geographies.
Compound flood risk further complicates the regional hazard outlook. The simultaneous convergence of heavy upstream rainfall, storm surge, and elevated sea levels multiplies impacts in river deltas and urban centres.
Modelling of the Mekong Delta under compound scenarios demonstrates that inundated areas and water depths expand significantly compared with single-driver flood models.
The persistence of elevated sea levels reduces drainage efficiency, ensuring that recovery lags as floodwaters linger. This systemic characteristic makes La Niña a multiplier of preexisting vulnerabilities. Effective disaster risk reduction requires integrated strategies combining hydrological forecasting, dam operation protocols, and proactive relief prepositioning.
Strategic readiness during OND 2025 will determine the extent of avoided losses in the short-lived but high-stakes La Niña episode forecast for 2025-2026.